Nice info mate! I didn't realise the front and rear units were different. may have another play soon,but i think i've just gotten used to mine now. probably around 20% better than standard,but not worth the £160 i spent on the speakers and adaptersAfter fitting my DNX520VBT and Infinity 652.9i's in the front i noticed a lack of Bass too. however disabling the Built in Tweeters is a must-do and i have now tweaked the Audio settings/xover settings on the new HU to allow a more rich bass in the front of the Altea, Sound now is Very good. Tones across the full range are clear/crisp and not distorted at higher Volume levels. I have a couple of small Vibration issues which i think i can solve by a glue gun to the Speaker Adapters/sound proofing.
The rear speakers however are crap. ETKA list the standard speakers in my car as Mid tones in the rear and Bassin the Front. I may see if i can fit the original "bass" into the back to give some extra bass from behind.
I have put up a review of the DNX over here for anyone interested: